The present invention relates to cast iron with improved ductility and high temperature properties. More specifically, the present invention relates to cast iron alloys which contain relatively high levels of aluminum, yet are not brittle.
Cast iron alloys, and parts cast from the alloys are subject in use to an ever increasing range of challenging environments. Such parts must operate at high temperatures and withstand temperature cycling between periods of use. The parts must have good oxidation resistance and be resistant to mechanical and thermal fatigue and oxidative cracking.
Turbocharger turbine housings and exhaust manifolds are examples of components which are subjected to challenging environments. Typically the in use temperatures for such components are on the order of 900xc2x0 C. or greater. For example, with today""s high performance engines it is not uncommon to see in use temperatures reach 1000xc2x0 C. and higher.
In addition to exhibiting high temperature capabilities, iron alloys generally must be easy to handle, machine, and abrasively clean at room temperature. Further, alloys are desired which are less brittle at room temperature and at the temperature of use.
Cast iron alloys which are currently known generally have a relatively high mechanical strength but tend to have a relatively low ductility, i.e., the components manufactured therefrom tend to be somewhat brittle. It is desirable to increase the mechanical strength of the alloy at elevated temperatures, while maintaining or improving the ductility at both room and elevated temperatures.
In an attempt to address the above described concerns of high strength at elevated temperature good thermal fatigue resistance, good oxidation resistance, and reasonable ductility, industry has developed ductile cast irons with high silicon and high molybdenum, commonly known as SiMo alloys. The SiMo alloys exhibit improved high temperature strength and thermal fatigue resistance over other ductile cast irons, as well as improved high temperature oxidation resistance. However, these property improvements are accompanied with a somewhat lower ductility at ambient temperatures and reduced machinability than other ductile cast irons. Despite these improvements in high temperature strength and thermal fatigue resistance and oxidation resistance, further enhancement of these properties would be desirable, in an alloy with acceptable ductility. The high oxidation rate at the high temperatures is especially a problem in parts such as exhaust manifolds and turbocharger turbine housings, where as indicated above the in-use temperatures can reach 1000xc2x0 C. and higher. In addition, cast irons in these applications are also subject to thermal fatigue cracking. This is due at least in part to the fact that the ferrite-austenite phase change temperature of these alloys is typically below the temperature of use. Therefore, in use the part is cycled up to temperatures associated with engine operation and then back down to room temperature. The part undergoes the ferrite austenite phase change upon heating and again upon cooling. This continued thermal cycling and associated phase transformation is said to contribute to thermal fatigue in the part which, in time, leads to cracking.
In addition to SiMo alloys, iron alloys that contain aluminum are also known. Such high aluminum iron alloys tend to have better high temperature oxidation resistance than conventional iron alloys. Furthermore, the aluminum content of the iron shifts the ferrite-austenite phase change temperature to higher temperatures, with the shift being greater as more aluminum is added. This is desirable for applications such as exhaust manifolds because it may be possible to formulate an alloy with a phase change temperature above the use temperature. Components formed from such alloys would not be subject to fatigue cracking associated with phase changes developed through thermal cycling. As higher aluminum contents push the phase change temperature to higher levels, higher use temperatures may be reached. Interestingly, it has been observed that as one goes to higher aluminum content in an iron alloy, the alloys become harder and more brittle at room temperature. Parts cast from the brittle iron alloys are more difficult to machine or to abrasively clean because of their hardness, and they are subject to fracture during subsequent processing and handling because of their brittleness.
According to U.S. Pat. No. 5,236,660 to Reynaud, cast iron alloys containing silicon and aluminum have acceptable high temperature properties and as such can replace the more expensive high nickel alloys in high temperature applicants. However, the Reynaud patent discloses data indicating that alloys containing high levels of those elements are too brittle for use in such applications. Specifically, Reynaud defines an empirical parameter, the xe2x80x9csilicon equivalentxe2x80x9d, as equal to the silicon level (in wt %) plus 80% of the aluminum level (in wt %) and states that when the silicon equivalent of the alloy is greater than 7.1% articles cast from the alloy are too brittle for high temperature use. Further, within the limits of the silicon equivalent as defined, the silicon level can range from 3.9 to 5.3%, but the aluminum level can be no greater than 2.5%.
Therefore, it would be desirable to provide an iron alloy with a high level of aluminum and combining properties of high mechanical strength and high ductility. It would also be desirable to provide parts cast from such an alloy that could be readily machined and abrasively cleaned at low temperature, and could also withstand oxidation at high in-use temperatures. Because the ferrite-austenite phase transformation temperature would be above the in-use temperature in many engine applications, such parts used under these conditions would be more resistant to fatigue cracking associated with any transformation-induced thermal cycling.